Social psychology is the scientific study of how people's thoughts, feelings and behaviors are influenced by the actual, imagined or implied presence of others. In this definition, scientific refers to the empirical investigation using the scientific method. The terms thoughts, feelings and behavior refer to psychological variables that can be measured in humans.
Salamu Alaikum. I came across this fascinating article which describes in detail the contributions of a variety of scholars to the field of psychology. I hope that this can help answer some of your questions.
Al-islam.org also has some information on this specific scholar : https://www.al-islam.org/history-muslim-philosophy-volume-1-book-3/chapter-32-fakhr-al-razi